1. Field of the Invention
The field of the invention is that of amplifiers and in particular microwave power amplifiers with a variable output power and a predetermined frequency band (allocated channel).
2. Description of the Prior Art
Amplifiers of this kind are used in cellular mobile radio systems, for example, including the GSM (Global System for Mobile communications) and TETRA (Trans-European Trunked Radio) systems in which each mobile station is allocated a transmission channel (frequency band).
In each GSM mobile station, for example, an amplifier has to output an amplified signal in the allocated channel, the output power varying in steps of 2 dB over a range of at least 24 dB to allow optimum operation of the system.
In the TETRA system an amplifier has to output an amplified signal in the allocated channel but the output power varies in steps of 5 dB between the maximum power for the mobile station and 15 dBm.
Microwave amplifiers have transfer functions in which the degree of non-linearity is proportional to the output power. This non-linearity generates intermodulation products which broaden the spectrum of the amplified signals.
This broadening of the spectrum compromises correct operation of the radio system in which the amplifiers are used. The amplified signal spills over from the allocated channel (out-band transmission) and represents an interference signal to adjacent channels.
In mobile radio applications the maximal out-band send level (i.e., the maximal interference send level) is specified and enforced to limit interference between channels and guarantee correct operation of the mobile radio system.
There are various ways for limiting out-band output from an amplifier.
A first solution uses the amplifier at much lower than its rated capacity, so that it operates at all times in a linear region (the lower the power level, the less non-linearity is observed). An amplifier rated at 100 W, for example, is biased for this maximum power but is operated at only 10 W. In other words, the amplifier is biased so that linearity constraints are met for the maximal output power and this bias is retained regardless of the actual output power.
This first prior art solution has a major drawback, namely its very low power efficiency (the ratio between the power of the RF output signal and the power drawn from the power supply).
Operating the amplifier at below its rated capacity entails high power consumption and dissipation of a large quantity of energy, and therefore the use of large and heavy heatsinks. In mobile radio systems each mobile station is a portable battery-powered device incorporating an amplifier. Consequently, high power consumption by the amplifier reduces battery life and the use of large and heavy heatsinks increases the overall size and the weight of the portable device.
A second prior art solution to the problem of limiting out-band output by an amplifier is to add to the amplifier a linearization device to limit broadening of the spectrum. The second solution improves power efficiency but does not optimize it.
It is clear that it is preferable to have good power efficiency combined with good linearity. As these two parameters are mutually contradictory, however, linearity in conformance with out-band send level constraints is always achieved to the detriment of power efficiency.
An object of the invention is to remedy these various drawbacks of the prior art.
To be more precise, a main object of the present invention is to provide an amplifier bias control device which, by adjustment of the bias, obtains the best efficiency/linearity trade-off regardless of the amplifier power output level.
Consequently, one object of the invention is to improve the battery life of portable devices including an amplifier of this kind and either to reduce the overall size and weight of the portable device by enabling a reduction in the size of the energy dissipating heatsinks or to reduce the size of the batteries.